Sirius rolled and came to his feet in a crouch. In all the chaotic melee, with the smoke and the sparks and the ghosts and the people, all he could see was Bellatrix. "Your turn this time, Trixie!" he called back, but by now she was too far from the Veil for him to make good on it. He sent a slashing hex instead, and was gratified to see a spray of blood from her hand, though he would have rather it was her wand hand, or better yet, her neck.
In his single-minded attack, he neglected to watch for other threats, and a Body-Bind curse hit him from the side, sending him crashing to the ground from his own forward motion.